Isolating the spectroscopic signature of a hydration shell with the use ofclusters: Superoxide tetrahydrate

Cluster spectroscopy, aided by ab initio theory, was used to determine the detailed structure of a complete hydration shell around an anion. Infrared spectra of size-selected O-2(-.) (H2O)(n) (n = 1 to 4) cluster ions were ob tained by photoevaporation of an argon nanomatrix. Four water molecules are required to complete the coordination shell. The simple spectrum of the te trahydrate reveals a structure in which each water molecule is engaged in a single hydrogen bond to one of the four Lobes of the pi* orbital of the su peroxide, whereas the water molecules bind together in pairs. This illustra tes how water networks deform upon accommodating a solute ion to create a d istinct supramolecular species.